Nano storage Hard drives could one day be the size of rice grains, powering music players so small they would fit inside your ear.

Scientists at IBM and the German Center for Free-Electron Laser Science have built the world's smallest unit of magnetic storage, using just 96 atoms to create one byte of data. Conventional drives require a half a billion atoms for each byte.

The advance could lead to tiny hard drives able to store 200 to 300 times more information than they can today. Just imagine an iPod Touch that held 12.8 terabytes of music.

"An effect that is common in nature can produce this information storage idea," says Sebastian Loth of CFEL, lead author of the research, which is being published today in the journal Science.

The natural phenomenon Loth is referring to has to do with the way electrons spin inside an atom. Modern hard drives rely on magnetic materials such as iron, where electrons all spin in the same direction perfectly aligned with each other. The drives work by reading the magnetic states of small regions on a disk and using an external field to write to them.

But these so-called ferromagnetic materials can only be shrunk down so far. If the magnetic regions get too close to each other, their magnetic fields interfere with each other and make it difficult to accurately store data.

"This is a big problem if you want to pack in the magnetic density," says Loth.

But with materials that are not magnetic, known as antiferromagnetic materials, the electrons spin in opposite directions from one another and are magnetically neutral.

"Antiferromagnetic regions don't have a magnetic field so you can pack them closer," Loth says.

In fact, the scientists were able to squeeze bits into a space just one nanometre apart.

Built atom by atom

The team assembled the tiny hard drive from the atom up, using a special tool known as scanning tunnelling microscope, or STM. They carefully placed atoms into rows of six atoms each. Two rows were enough to store one bit of information. Eight pairs of rows amounted to one byte of data.

Each pair of rows has two possible magnetic states, representing the classical 0 and 1 of binary computer data. An electric pulse from the STM tip flips the magnetic configuration from one to the other. A weaker pulse was used to read it.

"What this shows is you have all the ingredients for storing information on an antiferromagnetic grain," says Dr Matthias Bode, an experimental physics professor at the University of Würzburg, who was not involved in the research.

It will be some time before this technology is used in a hard drive for a computer, as there are a few problems that still have to be overcome. First, this hard drive was built atom-by-atom, using an STM - an impractical and slow method for manufacturing.

Secondly, the storage of the information, the magnetic state, is only stable at very cold temperatures, about 5 degrees above absolute zero. Warmer than that and the spins of the atoms get jostled.

Bode says that finding a material that works at room temperature isn't impossible. What material will work, however, remains to be seen.

Loth notes there are lots of other materials to experiment with that are known to hold antiferromagnetic states at room temperature. "This isn't like superconductors, where we are looking for ways to boost the critical temperature," says Loth. "We know that antiferromagnetic materials are stable."

This work is also important because it demonstrated for the scientists how few atoms they could use before the effects of quantum mechanics took over. It turns out that twelve atoms are the minimum number required. Fewer than that and quantum effects begin to mess around with the stored information.